Mid-Infrared Spectroscopy of High-Redshift Submillimeter Galaxies: First Results

We present mid-infrared spectra of five submillimeter galaxies at z=0.65-2.38 taken with the Spitzer Space Telescope. Four of these sources, at z<~1.5, have strong PAH features and their composite spectrum is well fitted by an M82-like spectrum with an additional power-law component consistent with that expected from AGN activity. Based on comparison with local templates of the 7.7 μm PAH equivalent width and the PAH-to-infrared luminosity ratio, we conclude that these galaxies host both star formation and AGN activity, with star formation dominating the bolometric luminosity. The source at z=2.38 displays a Mrk 231-type broad emission feature at rest frame ~8 μm that does not conform to the typical 7.7 μm/8.6 μm PAH complex in starburst galaxies, suggesting a more substantial AGN contribution.